Washington: Still suffering from the Monday morning blues? You may be in the minority, as most people in the world feel upbeat in the morning before becoming grumpier later in the day, finds to a new study on Twitter messages.

The study, which examined the contents of more than 500 million tweets sent in 84 countries over two years, found that people wake up happy but become grumpier as the day wears on and rebound in the evening, with a peak right before bedtime.

The study, published in the journal Science, also showed that people are happier from December to late June, when days gradually lengthen in the Northern Hemisphere.

The discovery, the researchers said, will interest those who are trying to understand how circadian rhythms and other natural influences shape our states of mind.

"We now have the ability to view societies at a massive scale using the Internet," said study leader Scott Golder, a graduate student in sociology at Cornell.

"This will open up opportunities for social scientists," Golder was quoted as saying by a news daily.

According to the research, optimism is reborn with each new day and slowly erodes as people work, study and go about their quotidian affairs. Their mood lifts as they head home to friends, family, entertainment and beer, they said.

For the study, the researchers wrote a computer programme that sampled all Twitter accounts created between February 2008 and April 2009, gathering up to 400 messages or tweets from each account.

The programme compiled more than half a billion tweets, some of which expressed positive feelings like enthusiasm, delight and alertness, while others indicated negative emotions like distress, fear, anger, guilt and disgust.

Positive language accounted for about six per cent of all words used at the highest point but dropped to five per cent during working hours while negative messages rose slowly throughout the day. They found that positive-mood tweets peaked twice a day throughout the world, early in the morning and again near midnight. The morning peak came later on weekends, presumably because people slept in.

That the cycle was similar on weekdays (when pressures like work deadlines and school exams pile up) and weekends (when most people are more relaxed) showed that sleep schedules and circadian rhythms were important influencers of mood, regardless of day-to-day stresses, the authors reported.

They also found that, on average, moods improved as the days lengthened in the spring and worsened as days shortened in the fall.

Dr Nicholas Christakis, a medical sociologist at Harvard University who was not involved in the research, said that such insights might seem underwhelming at first: "Do we need scientists to tell us our mood is better in the morning?""This should reassure people the method is not crazy. You want to sort of calibrate the instrument, and I think these results are a good indication that the instrument is telling you reasonable things," said Duncan Watts, a sociologist and researcher at Yahoo Labs.

Other experts, however, wonder whether just knowing a person's or a population’s emotional state tells you much.

"The real problem with this method is that you don't know what the people are doing," said Jonathan Gershuny, an Oxford University sociologist said.